External Refractive Index Changes Research Articles

An Antiinterference Temperature Sensor Based on Mach-Zehnder Interferometer Using Kagome Hollow-Core Photonic Crystal Fiber

An anti-interference temperature sensor based on Mach-Zehnder interferometer (MZI) is proposed, which is formed by successively fusing a homemade Kagome Hollow-Core photonic crystal fiber (HC-PCF) with two no-core fibers and two single-mode fibers. Due to the anti-resonant reflection optical waveguide properties, the interference between hollow-core mode and glass mode is achieved in Kagome HC-PCF, which significantly simplifies the overall sensing architecture and maintains better stability of the sensing signal. Using Kagome HC-PCF length of 497 μm, the maximum temperature sensitivity of the proposed sensor is obtained as 38.39 pm/°C. Meanwhile, it exhibits a low sensitivity of 0.5041 pm/με to ambient stress variation, -0.28 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to curvature change and 9.35 nm/RIU to external refractive index change. This MZI temperature sensor has the advantages of compact structure, easy fabrication, low cost, small size, high sensitivity and anti-interference capability, which can be expected to be applied to temperature sensing in complex environments.

Application of microstructured fiber sensor in the field of temperature detection

A fiber temperature sensor based on no core fiber-few mode fiber-no core fiber (NCF-FMF-NCF) is proposed. It consists of two segments of NCF and a segment of FMF, with the NCF fused at both ends of the FMF. Meanwhile, the lengths of the NCF and FMF were optimized by simulation simulations and experimental validation. The results show that the sensor has a high sensitivity to the external refractive index (RI) changes, and enables a wide range of ambient temperature measurement. A sensitivity of 0.09445nm/? was obtained in a temperature range of 25-70?. The sensor has the advantages of high stability, good linear fit and simple structure.

Ultrasensitive and Broadband Optical Toroidal Modes in all‐Dielectric Nanostructures

AbstractDynamic toroidal dipole (TD) with its peculiar characteristic of broken space‐inversion and time‐reversal symmetries plays an important role in the fundamental physics of light–matter interaction. Here, TD metamaterials comprised of amorphous silicon nanopillar arrays embedded in spin‐on‐glass layer are experimentally demonstrated. Upon normal incidence of plane wave, the transverse toroidal moment and the associated anapole‐like state are excited in optical regime. The strong TD response stems from a complete head‐to‐tail configuration of the magnetic dipole moments within each individual nanopillar. Both the experimental and simulation results show that such TD mode sustains a large structural tolerance and can be spectrally tuned by elongating the cylindrical axis perpendicular to the light polarization, corresponding to a cross‐sectional variation from circular to elliptical shapes. The excited TD mode is found to exhibit ultrahigh refractive index sensitivity compared to other multipoles, resulting in a sensitivity of 459 nm (470 nm) per external refractive index change in the experiment (calculation). This approach provides a simple and straightforward path in realizing toroidal metamaterials and establishes a new flat‐optics platform for realizing active metadevices, sensors, and nonlinear nanophotonics.

Dual-parameter sensing of refractive index and pressure by dual-polarization microresonators

We propose a dual-polarization microring resonator that can simultaneously measure refractive index and pressure. Since the transverse electric (TE) mode and the transverse magnetic (TM) mode of a silicon waveguide have different energy distributions, TE and TM polarization have significant differences in sensitivity to environmental refractive index and pressure changes. Different responses to external refractive index and pressure changes can be obtained by simultaneously exciting TE and TM modes in a microring resonator. The refractive index sensitivities obtained in the experiment were 132.97 and 40.54 nm/RIU; the pressure sensitivities were 1.41 and 1.59 pm/KPa, respectively. By inversion of the second-order sensitivity matrix, it is verified that the effective refractive index and pressure response can be obtained simultaneously through a single measurement to realize the dual-parameter sensing of the surrounding refractive index and the pressure of the device structure.

Self-referred microcavity-based fused-fiber fabry-perot refractometer

In this paper, a fiber-optic Fabry-Perot microcavity sensor is presented based on splicing a catastrophic fused fiber (CFF) to a single-mode fiber (SMF) for the simultaneous measurement of the refractive index (RI) and temperature. CFF formed by the catastrophic fuse effect consists of a periodic void in the fiber core that creates a teardrop shape or elliptical air microcavity after splicing. By simulation, the elliptical microcavity sensor is selected to measure external RI changes by Fast Fourier Transform (FFT) analysis. The RI sensitivity was obtained to be −2.5 ± 0.0074/RIU with a resolution of about 4 × 10−5 RIU. Also, the experimental results showed that temperature is obtained independently from the FFT parameter without being affected by the RI. The sensor temperature was determined by tracking the wavelength shift of the interference spectrum. Consequently, the temperature sensitivity of 7 pm/°C in the range of 20 °C to 95 °C was achieved. Therefore, the RI and the temperature were simultaneously measured by analyzing the spatial frequency and wavelength shift. The proposed dual-microcavity FP sensor can compensate for any power fluctuation in the input light source.

Refractive Index Insensitive Triple Cladding Quartz Specialty Fiber Temperature Sensor With D-Type Microcavity Structure

A refractive index insensitive triple cladding quartz specialty fiber temperature sensor with D-type microcavity structure is proposed. It consists of a triple cladding quartz specialty fiber(TCQSF) with a D-type microcavity structure fused between two sections of single-mode fiber(SMF). Because the SMF and TCQSF mode fields do not match, the excited modes form a Mach-Zehnder(M-Z) interferometer, resulting in a mode interference phenomenon. Experimental results show that in the range of 30-90°C, the transmission spectrum has a red shift phenomenon, the temperature sensitivity is 138.03 pm/°C, and the wavelength shift has a good linear relationship with temperature change. When the external refractive index changes in the range of 1.3350-1.3466, the sensor has a low sensitivity to the refractive index, which can avoid the cross-sensitivity of simultaneous measurement of temperature and refractive index. The sensor is not only simple to fabricate and high in sensitivity, but also has good repeatability and stable response, and has wide application prospects in the field of sensing.

Polished photonic crystal fiber refractive index sensor based on surface plasmon resonance

A polished photonic crystal fiber sensor based on surface plasmon resonance for refractive index (RI) detection is proposed and analyzed by finite element analysis. The design of polishing can greatly increase the birefringence of photonic crystal fiber and ensure a uniform coating film, thus improving sensor performance. The outer air of the structure is arranged in half of an octagonal shape, and the inner layer is arranged in a quadrilateral shape, which can effectively improve coupling strength in the y -polarization direction. The sensor works mainly in visible and infrared wavelengths with a detection range of 1.33 to 1.38. Through analysis of external RI changes, pore structure parameters, and silver mold thickness, the maximum sensitivity is 14800 nm/RIU, maximum amplitude sensitivity is 766.7008 R I U − 1 , resolution is up to 6.7568 × 10 − 6 R I U , and maximum figure of merit is 548.1481. The sensor is suitable for biomedicine, environmental monitoring, chemistry, and other fields.

Improved optical fiber Mach-Zehnder high-sensitivity refractive index sensor

A simple, highly sensitive, and all-fiber four-tapered Mach-Zehnder interferometer (FTMZI) refractive index (RI) sensor was proposed and experimentally demonstrated, which was fabricated by simply and repeatedly discharging a small section of single-mode fiber (SMF). Multiple tapers excite the high-order mode of the fiber cladding, making it sensitive to changes of external RI. The sensor shows a high sensitivity of 987.9 nm/RIU when the external RI changes from 1.330 to 1.346. Furthermore, the thermal response experimental results show that the temperature sensitivity of the sensor is 19.2 pm/°C, and the temperature crosstalk is only 0.00002 RIU/°C, making it a suitable candidate for temperature independent RI sensing.

Graphene Quantum Dots Assisted Optically Tunable In-Core Mach-Zehnder Interferometer

An optically tunable in-core Mach-Zehnder interferometer (MZI) based on the photoexcitation dynamics of nitrogen-doped graphene quantum dots (NGQDs) is demonstrated. The tunable MZI is fabricated by depositing NGQDs into a femtosecond laser ablated microgroove in the core. Combining both the high sensitivity to the external refractive index change of the MZI itself and the quantum confinement effect in quantum dots, a tuning range of ~90 nm is achieved as the pump power density of an ultraviolet (UV) LED varies from 0 to 41.1 mW/cm2, which accounts for 85% of the free spectrum range (FSR) of the MZI with a length of $205~\mu \text{m}$ . Such an optically tunable MZI with high integration, tuning efficiency and wide FSR can be suitably used in edge filter demodulation and tunable laser.

High-sensitivity salinity measurement sensor based on no-core fiber

A compact, stable, low cost and high sensitivity fiber salinity sensor based on single mode fiber-single mode fiber-no-core fiber-single mode fiber-single mode fiber (SSNSS) structure is proposed and demonstrated. It is proved by experiments and simulations that the sensor has high sensitivity to external refractive index (RI) changes, so as to achieve the purpose of gauging the seawater salinity. When the RI of surrounding media in sensing area changes, the absolute value of sensitivity for the refractive index measurement can be above 16,000 nm/refractive index unit (RIU) that is higher than other fiber salinity sensors as we known. In the experimental study, a fiber salinity sensor with the sensitivity of up to -3.42 nm/‰ was achieved when the salinity varied between 0‰ and 40.001‰, and the sensor showed a good linear response.

High-Efficiency Spiral Zone Plates in Sapphire

Here, we report embedded spiral zone plates (SZPs) processed by tightly focused femtosecond pulses, inducing a refractive index change in a sapphire crystal. The change in the index is investigated both experimentally and theoretically and found to be 0.93% lower than that before the laser irradiation. Benefiting from the investigation of the photorefraction, a high diffractive efficiency of up to 78% is achieved for the SZPs. In addition, the SZPs are embedded under the surface of the sapphire, which ensures that the components possess stable optical properties in the case of external refractive index changes. The high-performance elements having a high diffractive efficiency and stable optical performance will have tremendous applications in optical communication and optical manipulation.

Gasoline Quality Sensor Based on Tilted Fiber Bragg Gratings

We report on the study of an intensity-based optical fiber sensor for gasoline quality monitoring. The sensor setup employs two Bragg gratings with different spectral responses to interrogate the optical response of a tilted Bragg grating. The sensor operation is based on the tilted Bragg grating sensitivity to external refractive index changes, which are translated as power variations by the interrogation scheme. Gasoline–ethanol solutions with concentrations ranging from 0% to 60% ethanol were used to demonstrate the sensor performance. The results allowed to estimate that the sensor is able, within its resolution limit, to detect ethanol concentration variations of 1.5% in gasoline–ethanol solutions and discriminate temperature variations of 0.5 °C. The all-optical sensor setup is compact and robust, making it a competitive alternative for the realization of fuel quality analyses in practical applications.

Optical investigations of electrochemical processes using a long-period fiber grating functionalized by indium tin oxide

The growing needs for fast and reliable sensing devices stimulate development of new technological solutions. In this work a new multi-domain sensing method is demonstrated where optical sensing device has been applied to enhance amount of data received during electrochemical analysis. Thin, optically transparent, high-refractive-index, and electrically conductive indium tin oxide (ITO) film was deposited using magnetron sputtering on the surface of the long-period gratings (LPG) induced in optical fiber. ITO serves as working electrode in electrochemical setup and as a coating that enhances sensitivity of a LPG to changes of external refractive index. All the measurements were conducted in a combined optical and electrochemical setup. Obtained results show capability of this approach for optical monitoring of both electrochemical processes and changes in optical properties at the ITO surface. The optical response of the sensor was strongly dependent on the voltage applied to the ITO-LPG working electrode and on composition of the electrolyte. Changes observed in optical spectra may be attributed to the alterations in optical properties of both ITO layer (refractive index changes) and electrolyte in the vicinity of electrode (double layer formation). The developed structures can be further used as a fiber-based sensors and biosensors with integrated opto-electrochemical readout.

Graphene-Oxide-Coated Long-Period Grating-Based Fiber Optic Sensor for Relative Humidity and External Refractive Index

Graphene oxide is a very attractive material for refractive index and humidity sensing due to its unique 2-D structure, which results in faster response times and improved sensitivity over alternative materials. In this paper, response of a graphene-oxide-coated long-period grating-based sensor to changes in relative humidity (RH) and external refractive index (as well as temperature to provide a correction for any changes) is reported. In fabricating the probes, an improved Hummer's method was used to synthesize the graphene oxide dispersion used as its basis, allowing coating of a functionalized long-period grating by using a dip-coating technique. A consistent and stable response of the resonance band intensity of the graphene-oxide-coated long-period grating was observed to the change in humidity, achieving a sensitivity of 0.15 dB/%RH with a linear correlation coefficient of 0.980 over the RH range from 60%RH to 95%RH, at room temperature (25 °C). A blue shift of the resonance band wavelength was recorded when the sensor was exposed to vary temperature conditions from 25 °C to 70 °C and the response was found to be linear, with a correlation coefficient of 0.997. When evaluating its performance as an external refractive index sensor, sensitivities of ∼17 dB/RIU in the lower refractive index region (1.33–1.38) and ∼55 dB/RIU over the higher refractive index region (1.40–1.45) were achieved. The graphene-oxide-coated long-period grating sensor probe performed well, showing a good stability and repeatability over a number of test cycles in the performance evaluation carried out.

Sensitivity enhancement of polymeric Mach–Zehnder interferometers by use of thin high-index films

A simple yet highly-effective means for sensitivity enhancement of polymer-based effective index sensors using tailored high-index thin films is presented. Simulation and experimental results show that an appropriately-placed PTFE/Ta2O5 thin film can lead to enhanced sensitivity of asymmetric Mach–Zehnder interferometers to external refractive index changes of factors between 3 and 8. The films are deposited on the polymer using ion beam sputtering (IBS) at low temperatures (<80°C) thereby avoiding detrimental heating effects on the substrate. It is shown that the limit of detection of the interferometer sensors is enhanced by a factor 20 (down to Δn≤10−4) using these films.

Design of all-polymer asymmetric Mach-Zehnder interferometer sensors.

New design concepts for all-polymer integrated optical Mach-Zehnder interferometers (MZIs) optimized for environmental sensing are presented. Fabricated using large-area printing techniques, these polymer-based components are designed for low cost fabrication while maintaining suitable sensitivities to external refractive index changes. One key aspect in their design is obviating the need for an "interaction window" over one arm of the interferometer, as is usually defined in semiconductor or glass components, but requiring additional lithography, deposition and etching steps not suitable for polymer printing technology. We thus employ an asymmetric MZI, and derive the form of structures with optimized sensitivity and operating characteristics. Using the derived design criteria, experimental verification using a chemical test system finally demonstrates the utility of the approach.

Study of lateral-drilled DBR fiber laser and its responsivity to external refractive index.

We report a lateral-drilled DBR fiber laser which contains a defective parabola-like opening inside the cavity fabricated by the CO2-laser exposure and study the laser responsivity to external refractive index (RI). Surrounding materials can readily reach the vicinity of the fiber core via the opening and interact with the laser mode. Research shows that the laser emission power mainly relies on changes of external RI while the lasing wavelength on temperature. The effects of structural parameters, pump power, and external refractive index on the RI responsivity of the device are demonstrated. The lasing threshold condition is also concerned. This work provides an opportunity for controlling emission characteristics of the DBR fiber laser through modification of external RI value, of which the results are valuable for the potential applications in optical sensing, tunable lasing, and etc.

Intermodal interferometer with low insertion loss and high extinction ratio composed of a slight offset point and a matching long period grating in two-mode photonic crystal fiber.

An all-fiber modal interferometer based on a long period grating (LPG) inscribed in a two-mode photonic crystal fiber (PCF) and a slight core-offset spliced end is proposed and demonstrated. The LPG is fabricated to realize energy coupling from LP01 core mode to LP11 core mode, and the two core modes will interfere at the slight core-offset spliced end. We analyze the impact of energy coupling efficiencies of the LPG and the output spliced end on the extinction ratio of the interference fringes. With an appropriate energy coupling efficiency matching condition, our modal interferometer can realize lower insertion loss and high extinction ratio. Moreover, the sensitivities of our interferometer to strain and temperature are investigated, and the good stability of this device to external refractive index change is also demonstrated. As an all-fiber interferometer made of pure silica, this device has great potential applications in high temperature sensing fields, especially in harsh conditions.

Etching Bragg gratings in Panda fibers for the temperature-independent refractive index sensing.

We demonstrate the evolution of the Bragg gratings inscribed in Panda fibers with chemical etching. The resonance wavelengths can blueshift with cladding reduction similar to the conventional counterparts. But the wavelength separation between the two polarizations is co-determined by the stress and the asymmetric shape effects. The fast and slow axes of the fiber can be reversed with each other and zero birefringence can be achieved by chemical etching the structure. When the stress-applying parts of the fiber are removed, the finalizing grating can be exploited for the temperature-independent refractive index sensing, since the modes corresponding to the two polarizations exhibit the dissimilar responses to the external refractive index change but the same response to temperature. Our device is featured with easy achievement, spectral controllability, and relative robustness.

An in-line optical fiber refractometer with porous thin film coating

An in-line refractive-index sensor is proposed and demonstrated. The sensor is based on Fresnel reflection modulated by Fabry–Perot interference, which consists of a section of multi-mode fiber (MMF) tip coated with porous dielectric multilayer. The measure is conducted by monitoring the shift of the interference fringe, which is correlated to different refractive indices. When the external refractive index (RI) changes from 1.333 to 1.443, the RI sensitivity is 135.5nm/RIU with good linearity. The miniature size, simple physical structure, small temperature dependence, linear response and ability for batch fabrication make the device attractive for chemical and biological sensing.